200-fold increase in HTML-attachment spam

Researchers at M86 have reported a significant increase in the amount
of spam sent with malicious HTML attachments, the volume of
which on some days was 200 times that on the first day of the year.

HTML, the mark-up language used to create web pages, is commonly used
in email to display various fonts and colours and to embed images. All
modern email clients are capable of displaying HTML emails, though it
is good practice for these to contain a text-part as well. A slight
modification to the emails means the HTML-part is seen as an
attachment that can be viewed in a web browser, rather than shown within the email client. It is this that is
being used in a large spam campaign, of which M86 believes the Cutwail
botnet is the perpetrator.

These particular emails - which either have the subject 'End of
August statement' or come with a 'Xerox scan' attached - contain an HTML
attachment in which, through obfuscated JavaScript, an
iframe is embedded. The Phoenix
exploit kit is loaded in the iframe, which attempts to infect the user
through exploits in various browsers and
plug-ins.

The tactic of infecting users via iframes and obfuscated JavaScript is
commonly used for drive-by downloads, mostly in
compromised legitimate websites. By using an HTML attachment rather
than a website, this kind of attack is less likely to be picked up by
web filters, while spam filters may
not attempt to de-obfuscate the JavaScript, thus making it less likely
for URL blacklists to block the emails.

HTML-attachment spam made the news last month when it was said that
users could be infected without opening the attachments. While it is not
impossible for
an email client to open the attachment and render the JavaScript, either through a bug or through bad design, it
seems unlikely for this to happen. We have not found evidence of an
email client with that property.

More at M86here.
The original story on the emails allegedly infecting users without the need to
open an attachment is at elevenhere,
with comments from Sophos's Naked Security blog here.

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